QT interval in the electrocardiogram (ECG) is defined as the time interval between the start of Q wave and end of T wave in the electrical cycle of heart. The QT interval is a measure of the combination of cardiac repolarization as well as cardiac depolarization. It comprises of QRS complex as well as ST complex in the ECG. Moreover, it is believed that the delay of ventricular conduction is affiliated with the length of the QT interval. Normally, the QT interval is between the 0.36 seconds and 0.44 seconds which means that 9 to 11 boxes of ECG paper.
A QT interval is measured by the ECG monitor automatically and can be measured manually with the help of ECG machine. It is recommended by several health professionals to measure it manually, because heart rate of more than 100 beats/minute will suppress the QT interval. One small box of the ECG paper represents 0.04 seconds. Boxes will be calculated from start of the Q wave and end of the T wave. When determining the QT interval, it is necessary to measure it from the end of T wave rather than U wave. Sometimes U wave is mistaken with T wave while measuring QT interval. Therefore, it is preferred to use lead aVL while measuring the QT interval. In this lead, U waves are least prominent as compared to other leads.
It is not easy to decide whether the QT interval is normal or not because the duration changes according to the heart rate of the patient. The faster the HR, slower will be the QT interval. Therefore, it is recommended to calculate corrected interval of QT by using the formula
(QTc= QT /√RR)
Where QT represents interval of QT and RR represents the RR interval.
The QT interval represents the action potentional of myocytes of ventricles. The action potential is movement of ions across the membrane of cell via channels that are constituted of complexes of protein. Poor working of these channels will either decreased outward current of enhanced the inward current. It will lead to increase the duration of action potential and therefore cause prolongation of QT interval. The prolonged length of the interval of QT can be associated with certain fatal rhythms such as TdP (Torsades de Pointes). Wide variety of drugs are there that can cause the prolongation of QT interval. These medications include antipsychotics, antihistamines, antidysarrythmias and many more. Torsades de Pointes is the most dangerous arrhythmia which is characterized by the increased in QT interval and several categories of drugs have cause this. That’s why, these drugs have been removed from the market. People who use these drugs will end up with arrhythmias such as premature ventricular contraction and many more that cause fatal outcomes such as death. Prolongation of QT interval can increase the length of stay in hospital. Use of these drugs have been banned from the market because of the safety concerns.
Some drugs can cause prolongation of QT interval or Torsades de Pointes alone, while some drugs cause this in drug interaction situation. Situation of drug interaction have caused the inhibition of metabolism and it made the removal of large number of medicines from the market such as astemizole and terfebadine.
Large number of drugs have been withdrawn from the market because they cause prolongation of QT interval or torsa de pointes. These drugs impact on the electrophysiology of the heart.